The present invention relates to a video switch circuit. The term "video switch circuit" as herein used is intended to mean a circuit which operates to turn on and off a composite video signal or to switch between plural composite video signals.
It is frequently required to turn on and off a composite video signal in a circuit for supplying a composite video signal to a video signal receiving device. If, in this connection, the composite video signal is merely turned off, or the application of the composite video signal is completely suspended with no signal at all outputted, synchronization is lost in the video signal receiving device since the synchronizing signals are carried by the composite video signal. When the video switch circuit is later turned back on to again supply a composite video signal, the synchronization remains unstable for some period of time. Accordingly, it is necessary to supply a synchronizing signal to the video device even if the video switch circuit has been turned off. In other words, when the video switch circuit is turned off, it is necessary to still provide the synchronizing signal at the output of the video switch circuit.
In order to meet this requirement, the synchronizing signal in this case must be compatible in signal voltage level with the synchronizing signal of the composite video signal supplied to the video signal receiving device. Video switch circuits of the prior art satisfying such a requirement unavoidably were intricate in construction.
In view of the foregoing, an object of the invention is to provide a video switch circuit having a simple construction in which the above-described difficulties have been eliminated.